The technique of microiontophoresis was utilized to study the effect of dopamine administration on the activity of neurons in the region of the subthalamic nucleus. Five-barrel microelectrodes were placed stereotaxically in the subthalamic nucleus in urethane-anesthetized rats and dopamine was applied iontophoretically to both spontaneously active cells and to cells whose activity was maintained by continuous iontophoretic application of glutamate. Acetylcholine and haloperidol were also applied to some cells. Both increases and decreases in the spontaneous firing of neurons in the vicinity of the subthalamic nucleus were observed. In addition, some of these responses could be blocked by haloperidol. Relatively few responses (increases or decreases in firing rate) to acetylcholine were observed.